This invention relates to slurry pipeline systems for transporting liquids containing solid or non-liquid materials. More specifically, this invention relates to the construction and operation of pipelines so that a pipeline will be capable of operating at below its design capacity without damaging the pipeline.
Pipelines are presently in wide use for transporting a variety of liquids, most commonly for transporting fossil fuels and gases. Pipelines are also used for transporting solids in the form of slurries in which fine solid particles are suspended in a carrier liquid, usually water. In the past slurry pipelines normally extended over relatively short distances.
No insurmountable problems are encountered when such a slurry pipeline extends over long distances and hilly terrain so long as it is operated at or near its design capacity.
Typically, slurry pipelines connect a mine or another bulk producing facility with a shipping terminal, a factory, or another user of the bulk material. Because of significant lead times, normally several years, between the initial start-up of the mine and/or the shipping terminal, factory, etc. and their full capacity operation, it is normally not possible to immediately operate the pipeline at or near its design capacity due to the lack of material required for full operation.
As the flow in slurry pipelines cannot normally be stopped because the suspended particles would then settle out and partially or fully plug up the pipeline, it is necessary to maintain a continuous flow in the pipeline. The pipeline must therefore be full of liquid and/or slurry material at all times. During below-capacity operation, this is usually accomplished by batching the slurry with another liquid, normally water, so that the pipeline is operated for a number of hours, say 40 hours, with the slurry, and thereafter for a number of hours, say 20 hours, with water.
It has been determined that this batch operation of the pipeline can be troublesome if the pipeline extends over hilly terrain and the slurry and the other liquid (hereinafter "water") have differing specific gravities. Normally, both conditions are encountered because overland pipelines almost always extend over varying elevations and because the slurries are normally a mixture of water and a solid, say coal or ore, which increases the specific gravity of the slurry over that of plain water by a factor of as much as 2. The word "liquid" as used hereinafter is meant to include not only "pure" liquid, such as water, but also liquid in which solid material is suspended, e.g., slurry. Thus, a reference to a pipeline carrying batches of slurry and water is equivalent to a reference to a pipeline carrying batches of different liquids or liquid materials.
As a result, when the interface between a downstream water batch and an upstream slurry batch crosses over a high point of the pipeline, the much greater specific gravity of the slurry can cause a so-called "slack flow" of the slurry in the downgrade portion of the pipeline following the high point if the back pressure acting on the slurry is insufficient to counteract the static head of the slurry at the interface. In such an instance, the slurry ceases to completely fill the pipeline. Instead, it occupies a reduced, say 50%, cross-section of the pipeline only. Since the slurry throughput remains constant, the flow velocity of the slurry in the areas in which slack flow occurs is correspondingly increased. This increased flow velocity causes an abrasion of the pipeline wall and, depending on the type of suspended solid and the slack flow velocity, can lead to pipeline damage in a relatively short period of time, sometimes in as little as a few hours unless the condition is rectified.
The obvious solution to reducing pipeline damage from slack flow is to protect and reinforce the pipeline walls where slack flow may occur. This could be accomplished by increasing the pipeline thickness or lining the pipeline with rubber, polyurethane or similar materials. The problem with the obvious solution is that it is costly. Furthermore, the inevitable failure of the pipeline, the resulting need for an expensive shut down of the pipeline, and the necessary pipeline repairs, often at remote and almost inaccessible locations, are only postponed.